You need to know which device is the right one check with sudo fdisk -l

sudo su

/dev/sdXn should be replaced with the right device information, and depending on your installation you may have or have not a seperate /boot partition. If not /boot is a folder under / on the same partition as the filesystem root.

mount /dev/sdXn /mnt/ root partition

mount /dev/sdXn /mnt/boot/boot partition --> without extra /boot partition you do not need this step!